Effect Of Process Control On Microstructure And Properties Of Lanthanum Bismuth Titanate

Bi4Ti3O12 materials, which belong to the bismuth layer-structured family of compounds, has attract more and more attention because of a large remnant polarization, high piezoelectric coefficient and high Curie temperature. It has been found that the relative density of Bi4Ti3O12 ceramic sintered at 900°C by solid state method reaches over 90%, but the sintering temperatures of Bi3.25La0.75Ti3O12 samples to get the same relative density are above 1100°C. It was found that the sintering additive and a double sintering process both can increase the density of ceramics. The main aims of this thesis is to investigated the influence of SiO2 as the sintering additive and double sintering processes on the microstructure, ferroelectric properties and dielectric properties of Bi3.5La0.5Ti3O12 materials.We prepared Bi3.5La0.5Ti3O12 powders for sintering by the coprecipitation method as raw materials. Bi3.5La0.5Ti3O12 ceramics were sintered with amorphous silica as sintering additive by different sintering processes. The values of the relative density increase obviously when the second-phase sintering temperatures raise from 980°C to 1020°C and the relative density increases slowly when the second-phase sintering temperatures are above 1020°C. The analysis of the P-E hysteresis loops shows that the ferroelectric properties are improved as the second-phase sintering temperatures increasing, the vales of remnant polarization(Pr) is 9.8 μC/cm2 for Bi3.5La0.5Ti3O12 ceramic with SiO2 additive sintered at 1060°C for 2 hours, and the coercive filed(Ec) is 63.7 kV/cm.The results of microstructure observation show that the grain growth of Bi3.5La0.5Ti3O12 ceramic was improved by SiO2 as sintering additive, and increasing the maintenance time of sintering in the second-phase sintering process, the morphplogy of grains change from plate-like to ellipsoid-like. The values of the relative density, the dielectric properties and ferroelectric properties have a little change.The analysis of the P-E hysteresis loops show that the values of remnant polarization(Pr) is 7.1 μC/cm2 for Bi3.5La0.5Ti3O12 ceramic with 1.0% SiO2 sintered at 1040°C for 2 hours by double sintering method, which is less than that of the Bi3.5La0.5Ti3O12 ceramic without SiO2 additive prepared at the same process parameters, and the vaule of Bi3.5La0.5Ti3O12 ceramic without SiO2 additive by double sintering method is 11.0 μC/cm2. This illustrates that the 1.0% SiO2 as sintering additive does not improve the ferroelectric properties of materials. The SiO2 addictive can improve the growth of Bi3.5La0.5Ti3O12 ceramic grains, which leads to the differences in microstructure and ferroelectric properties compared with Bi3.5La0.5Ti3O12 ceramics sintered without SiO2 addictive.The dielectric temperature spectra analyses of Bi3.5La0.5Ti3O12 ceramics by different sintering processes show that the Curie temperature decreases as increasing the measurement frequency, and the maximum dielectric constant of Bi3.5La0.5Ti3O12 ceramic with SiO2 additives sintered at 1060°C for 2 hours by double sintering method reaches as much as 24400 at the Curie temperature measured at 1 kHz which is 11 times more than that of the material measured at 100 kHz.The dielectric temperature spectra analysis shows that the relationship between loss factor(tanδ) and temperature from 100°C to 450°C and the activation energy of oxygen vacancy is calculated from it. It illustrates that the value of activation energy due to oxygen vacancy ries from 0.66 eV of Bi3.5La0.5Ti3O12 ceramic without SiO2 additive to 0.83 eV of Bi3.5La0.5Ti3O12 ceramic with SiO2 additive, then the concentration of oxygen vacancy should decrease, but the loss peaks increase by double sintering process with SiO2 as sintering additive.